WASHINGTON (Reuters) - Joe Biden and Sarah Palin agreed that climate change is real, but differed on whether human activity was its root cause in Thursday's U.S. vice presidential debate.

Palin, the Republican governor of Alaska, acknowledged that human activities may play a role in heating up the planet, but also said natural cycles are part of the picture.

"I don't want to argue about the causes," she said in St. Louis. "What I want to argue about is, how are we going to get there to positively affect the impacts?"

To Biden, a Democratic senator from Delaware running with Sen. Barack Obama in the November 4 election, knowing the cause is critical to finding a cure.

"If you don't understand what the cause is, it's virtually impossible to come up with a solution," Biden said. "We know what the cause is. The cause is man-made. That's the cause. That's why the polar icecap is melting."

Palin's environmental policies have drawn criticism from green groups. She supports drilling for oil in the Arctic National Wildlife Refuge, which her presidential running-mate, Sen. John McCain, opposes.

She said McCain favored an "all-of-the-above" approach to battling climate change, including the use of alternative fuels and conservation.

Biden said McCain has voted 20 times in the 15 years against funding alternative energy sources including solar, biofuels and wind power.

Obama, McCain and Biden have supported legislation to limit climate-warming carbon emissions, and on Thursday, Palin said she too favored this. But she also linked increased domestic oil production to the fight against global warming.

"As we rely more and more on other countries that don't care as much about the climate as we do, we're allowing them to produce and to emit and even pollute more than America would ever stand for," she said, when talking about reducing greenhouse gas emissions.

(Editing by Alan Elsner)

(For more Reuters information on the environment, see http://blogs.reuters.com/environment/)